The present invention relates to an image forming apparatus utilizing an electrophotographic system comprising an electrophotographic copying machine, a laser printer and a facsimile or a combination of these as OA (Office Automation) equipment. More particularly, the present invention relates to an image forming apparatus for use in obtaining reproduced images by primarily transferring a toner image formed on an image carrier onto an intermediate transfer belt once and then transferring the toner image onto a recording medium such as copying paper; to such an intermediate transfer belt; and to a process for the production of the same.
In a typical image forming apparatus utilizing the electrophotographic system, a uniform electric charge is formed on a photosensitive material as an image carrier made of inorganic or organic photoconductive material, then an electrostatic latent image is formed by means of a laser beam resulting from modulating an image signal, for example, and the electrostatic latent image is developed into a visible toner image by the use of electrically charged toner. Further, the toner image is transferred directly or via an intermediate transfer body to a recording medium such as copying paper in order to obtain a desired reproduced image.
The Japanese Patent Application Publication No. Sho 62-206567, for example, discloses an image forming apparatus employing a system in which a toner image formed on an image carrier is primarily transferred onto an intermediate transfer body and the toner image on the intermediate transfer body is transferred onto a recording medium.
With respect to transfer belt materials for use in image forming apparatus of such an intermediate transfer belt system, there have been proposed the following electrically-conductive endless belts made by adding an electrically-conductive agent such as carbon black to thermoplastic resin: for example, polyvinylidene fluoride (PVDF) (the Japanese Patent Application Publication Nos. Hei 5-200904 and Hei 6-228335); polycarbonate (PC) (the Japanese Patent Application Publication No. Hei 6-95521); polyalkylene terephthalate (PAT) (the Japanese Patent Application Publication No. Hei 6-149081); a blend of PAT and PC (the Japanese Patent Application Publication No. Hei 6-149083); and a blend of ethylene tetrafluoroethylene (TFE) copolymer (ETFE) and PC, a blend of ETFE and PAT, a blend of ETFE, PC and PAT (the Japanese Patent Application Publication No. Hei 6-149079).
Since the aforesaid electrically-conductive material made of thermoplastic resin such as PVDF, PC or the like is inferior in the Young's modulus; namely, 14000 kg/cm.sup.2 or lower, there exist problems arising from the increased deformation of the belt due to the stress applied during the driving of the belt, inability to obtain a high-quality transfer image with stability when the material is used for the intermediate transfer belt, and the inferior durability of the belt as cracks may occur in the end portion of the belt during its operation.
As a material excellent in mechanical characteristics, thermosetting polyimide resin, for example, may be mentioned. A seamless belt made of polyimide resin with carbon black dispersed therein was proposed in the Japanese Patent Application Publication No. Sho 63-311263, for example. The process of producing this endless belt comprises the steps of dispersing carbon black as an electrically-conductive agent in a polyamide acid solution as a precursor, flow-casting the dispersion on a metal drum and drying it, casting the film peeled off the drum at high temperatures to obtain a polyimide film, and cutting the film in lengths.
The generally-known process of forming such a film includes injecting into a tubular mold a polymer solution with carbon black dispersed therein, rotating the tubular mold at 1000-2000 rpm while heating it at, for example, 110-150.degree. C., forming the solution into a film by centrifugal molding, removing the semi-hardened film thus obtained from the mold and putting it on an iron core, and letting a polyimide reaction (ring closure reaction) progress at 300-500.degree. C. so as to effect proper hardening.
In the case of a rotary molding method such as the aforesaid centrifugal molding method, however, minute irregularities are formed on the surface of the film when the solvent is unevenly evaporated during the molding and proper hardening process. When the intermediate transfer belt made of the film like this is used to make secondary transfer, there also develops a problem causing minute inadequate transfer (hollow character) and the like to the image transferred to the recording medium. In order to obtain a smooth film, on the other hand, it takes hours to perform the molding and hardening process for evaporating and drying the solvent and hardening the polyamide acid, thus increasing the belt production cost.
Since the polyimide resin is superb in mechanical characteristics, the intermediate transfer belt is less deformed because of the pressing force when the paper is pressed by a bias roll against the belt. When the toner image is transferred electrostatically onto the intermediate transfer belt by acting an electric field thereon, the load applied by the pressing force of the bias roll is concentrated in the secondary transfer portion. Consequently, the toner image is aggregated and the charge density is raised, whereby the toner polarity may be varied as discharge is caused in the interior of the toner layer.
This factor develops a problem arising from poor image quality in that there appears a hollow character in which a line image is partially missing.
In the prior art examples, the electrically-conductive belt materials made of thermoplastic resin which is inferior in mechanical characteristics produce a great deal of deformation against the stress applied thereto while they are being driven and make transfer images of good quality unavailable with stability. On the other hand, the belt materials made of thermosetting resin excellent in mechanical characteristics are also problematical in that since the belts are less deformed against the pressing force of the bias roll in the secondary transfer portion, the aggregation of the toner image brings about poor image quality.
Further, a belt material of single layer structure made up by the rotary molding using electrically-conductive polyimide resin develops the problem of causing inadequate transfer to a transfer image as minute irregularities originating from variations in solvent evaporation are formed on the surface thereof. In the case of a belt material of double layer structure whose surface is covered with an elastic member such as silicone rubber, the problem is that a toner image is not transferred to a recording medium at the time of the secondary transfer because of the viscosity properties of the rubber material.